Material handling vehicles

ABSTRACT

Lateral bending moments which lift truck mast sections must oppose when loads are shifted laterally are reduced by disclosed chain-sprocket arrangements wherein a load carriage or other upper mast section is suspended from chains which extend laterally across the truck via sprockets carried by an intermediate mast section and which are tied to a lower mast section. One embodiment is disclosed which also reduces the bending moment by selectively relieving one of a pair of spaced apart lift cylinders to shift the effective support point. Application of the chain arrangement to various exemplary types of mast structures is illustrated.

Q United States Patent 1191 [111 3,830,342 Allen Aug. 20, 1974 [54] MATERIAL HANDLING VEHICLES 2,005,798 9/1970 Germany 187/9 [75] Inventor: Ralph E. Allen, Greene, N.Y.

Primary Examiner-Richard A. Schacher [73] Assignee. Eh; Raymond Corporation, Greene, Assistant Examiner james L Rowland Attorney, Agent, or Firm-Richard G. Stephens [22] Filed: Jan. 2, 1973 [21] Appl. No.: 320,670 ABSTRACT I [52] US. Cl. 187/9, 214/730 Lateral bending moments which lift truck mast 51] 1111.01 B66b 7/06, B66f 9/06 time must Oppose When leads are Shifted laterally are [58] Field 61 Search 187/9, 11, 17, 20, 26; redueed y diselesed ehain-spweket arrangements 182/63, 145; 214/75, 671. 374 730 731; wherein a load carriage or other upper mast section is 254 2 4 143 143 suspended from chains which extend laterally across I the truck via sprockets carried by an intermediate [56] References Cited mast section and which are tied to a lower mast sec- UNITED STATES PATENTS tion. One embodiment is disclosed which also reduces the bending moment by selectively relieving one of a f 143 pair of spaced apart lift cylinders to shift the effective 3235034 2,1966 fi support point. Application of the chain arrangement 34601700 8/1969 Kroiipa iili: i 14 730 various exemplary types mast Structures is FOREIGN PATENTS OR APPLICATIONS 20,377 10/1905 Great Britain ..187/9 trated.

36 Claims, 19 Drawing Figures PATENTED M11220 I974 saw 01 or 1o PATENIEBMFBZOIQH sum our 10 PATENIEB 1092mm sum near 10 Fl-G.6

MATERIAL HANDLING VEHICLES One conventional and widely used form of lift truck has a mast structure comprising a set of substantially rigid vertically extending upright members affixed to the base of the truck and spaced about several feet from each other on opposite sides of the centerline of the truck, a load carriage provided with forks or other load'manipulating means, and one or more hydraulic lift cylinders which move the load carriage and load up and down the mast structure. In typical applications the fixed upright members comprise channel shapes. The mast structure may comprise a single-section mast wherein the fixed upright members guide vertical movement of the load carriage, or it may include one or more intermediate telescopic sections which are also raised and lowered, while guided by the fixed upright members and in turn guiding the load carriage. The load is supported in cantilever fashion from the load carriage, with the carriage at the rear end of the load. The loaded carriage applies a forward bending moment to the mast structure. The one or two lift cylinders or rams are ordinarily spaced on or near the centerline of the truck, midway between the two upright members. The lift cylinders usually raise and lower one or two sprockets or pulleys, and a pair of lift chains connected between the load carriage and the base of the truck are typically routed over the sprockets, so that the load carriage is raised at twice the speed at which the lift rams are extended. Since the lift chains are ordinarily routed directly over the lift cylinders and attached to the carriage very near the centerlines of the cylinders, the effective support point is ordinarily located both laterally and longitudinally very near to the centerline of the single lift cylinder when only one lift cylinder is used, or located midway between two cylinders when two are used. The lift chains ordinarily are connected to opposite ends of a tension equalizing bar which is pivotally attached to the load carriage, so that its pivot axis defines the effective support point.

Since the load carriage is ordinarily centered laterally between the two spaced-apart uprights of the mast structure, the entire bending moment applied to the mast is ordinarily applied about a laterally extending axis, with little or no component of bending moment usually existing about a longitudinal axis, but a slight moment component occurring about a longitudinal axis. if the center-of-gravity (C.G.) of the load is not exactly centered on the pair of forks or other load-engaging means. The longitudinal cantilevering of the load tends to rotate the load carriage in a fork tips down direction, so that the load applies a force couple to the mast near the top and bottom of the load carriage where rollers on the load carriage engage either the fixed uprights of a single-section mast or a telescopic section of a multiple-section mast. This force couple tends to bow the mast section engaged by the rollers top forwardly. If the load carriage applies the force couple to a telescopic section, rollers on the telescopic section apply a similar force couple to the fixed uprights, via additional intermediate telescopic sections in some multiple-section masts, tending to bow all mast sections top forwardly. Because a bending moment about a laterally extending axis tends to bend a member in a longitudinal direction, such a moment is ordinarily termed a longitudinal bending moment, and because a moment about a longitudinal axis tends to bend a member in a lateral direction, the latter is ordinarily termed a lateral bending moment, and such terminology will be used herein. In the most widely used forms of lift truck, the longitudinal direction of the truck usually means the direction in which non-steerable wheels are pointed, and the fixed upright members of the mast structure are usually spaced apart from each other in a direction perpendicular thereto, in the lateral direction of the truck. In the discussion which follows, the term lateral will be used to mean the direction in which the fixed upright members of the mast structure are spaced from each other, irrespective of the placement or directions of the wheels and other portions of the truck which have no bearing on the invention.

The mast members ordinarily comprise I-beam or channel members having their flanges extending laterally, so that their greater bending moments of inertia resist the large longitudinal bending moments imposed by the cantilevered load, and thus so that any lateral bending moments are resisted by the lesser bending moments of inertia of such members. When the load carried on such a prior art truck is not laterally centered on the forks, the load tends to rotate the load carriage about a horizontal longitudinal axis, with, for example, rollers at the upper right and lower left corners of the load carriage being urged harder against the two l-beams of a telescopic mast section against which they roll, and the upper left corner and lower right corner carriage rollers tending to unseat from the two I-beams, the upper and lower ends of the pair of I-beams being rigidly tied together with cross-braces.

In addition to the above-described type of lift truck wherein the load carriage is always substantially centeredbetween the uprights so that only modest lateral bending moments occur, trucks are known in which means are provided to shift the load carriage laterally in either direction relative to the mast structure, usually for a distance slightly greater than the width of the load. The use of such lateral load-extending devices advantageously allows the truck to place and retrieve loads without having to make a right angle turn, thus saving aisle space otherwise required for turning the vehicle. Such a device, with the side of the truck adjacent a rack of shelves, will allow the truck operator to remove a load from one shelf of a vertical column of shelves or store a load without moving the base of the truck. In this latter type of truck where the load carriage may be shifted laterally relative to the mast structure, a problem arises in that the load imposes much greater lateral bending moments on the mast structure when the carriage is shifted appreciably from a laterally centered position, and in practice the lateral bending moment imposed typically may become of the order of one and one-half times the longitudinal bending moment. It will be seen that use of a device which allows a load to be shifted appreciably laterally requires that the mast members be provided with greatly increased bending moments of inertia to resist lateral bending. The fact that lateral bending moments become even greater than longitudinal moments suggests one conceivably might rotate the mast members so that their flanges extended longitudinally. Such an arrangement would still require substantial increases in both bending moments of inertia of each mast section, however, and hence use of heavier mast members. Such an arrangement would also require substantial revision of the usual roller connections between the load carriage and portions of the mast structure.

A primary object of the present invention is to provide improved lift truck mast arrangements for use with lateral load-extending devices, wherein lateral bending moments are significantly reduced, so that lateral load shifting does not require a large increase in the size and weight of mast structure, or stated alternatively, so that greater lateral shifting of a heavy load may be safely accomplished with mast members of a given size without undue bending or deflection of the mast. In accordance with one central concept of the present invention, lateral moments are significantly reduced by a crossed lift chain arrangement wherein one lift chain attached to the load carriage on a first side of the centerline defined by a lift cylinder is routed by sprocket or pulley means laterally across that centerline and tied to the truck base on the other or second side of the centerline, and wherein a second lift chain attached to the load carriage on said second or other side of the centerline is similarly routed by sprocket or pulley means laterally across the centerline and tied to the truck base on the first side of the centerline.

While one prior proposed system is capable of significantly reducing lateral bending moments, it is undesirably complex and expensive for some applications because it requires the use of two hydraulic lift cylinders and auxiliary control apparatus One object of the invention is to provide improved mast arrangements which reduce lateral bending moments even more effectively, but which require only a single lift cylinder and do not require complex hydraulic control appara- IUS.

As will be seen below, several forms of the present invention also incorporate hydraulic control techniques to further lessen lateral bending moments, and additional objects of the invention are to provide such systems which will be described below in detail.

Other objects of the invention will in part be obvious, and will in part appear hereinafter.

The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts, which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention reference should be had to the follow ing detailed description taken in connection with the accompanying drawings, in which:

FIG. 1 is a front-view diagram of a prior art lift truck having a two-section mast system illustrating various forces and moments imposed thereon by a laterally shifted load.

FIG. 2 is a front view diagram similar to FIG. 1 illustrating forces and moments in one form of the present invention.

FIGS. 3a and 3b are top and front elevation views, respectively, of one form of mast assembly constructed in accordance with the present invention.

FIG. 30 is a diagrammatic front view illustrating one possible modification of the assembly of FIGS. 30 and 3b.

FIGS. 3d and 3e are diagrammatic top and front elevation views illustrating a further possible modification of the assembly of FIGS. 30 and 312.

FIG. 4 is a diagrammatic front view diagram illustrating a form of the invention utilizing dual lift cylinders.

FIGS. 5a and 6b are graphs useful in understanding the operation of the invention.

FIG. 6 and FIGS. 6a-6d are diagrammatic front views illustrating a variety of different ways in which the invention may be applied to plural section masts.

FIG. 7 is a diagrammatic front view illustrating one manner in which the invention may be applied to an inside-out mast.

FIG. 8 is a diagram useful in understanding how the chain arrangement may be modified to incorporate sloping non-vertical courses of chain to provide beneficial effects.

FIG. 9 is a diagram illustrating one manner in which the chain arrangement of FIG. 2 may be modified to provide a greater reduction in lateral bending moments.

FIG. 10 is a front view diagram illustrating a further embodiment of the invention.

In the diagram of FIG. 1 representing a front view of a typical prior art truck, the ram 10 of a lift cylinder 11 mounted on the truck base 12 at centerline l3 vertically supports a telescopic upper mast section 14 comprising mast members and 14b interconnected by crosspieces 14c and 14d. Extensible or telescopic mast members such as 14a and 14b ordinarily comprise I- beam shapes having their flanges extending laterally, i.e., left-right in FIG. 1. Lift chain 15 reeved over pulley or sprocket 16 carried on the upper mast section 14 has one end (not shown) fixed to the truck base and the other end tied as shown to the load carriage 17, so that carriage I7 is raised or lowered at twice the speed of ram 10. If the load carriage includes a lateral loadshifting mechanism, so that the center-of-gravity of the load may be significantly laterally displaced from the lift chain, through a distance shown as dimension x in FIG. 1, a clockwise moment of magnitude W x will be seen to be applied to carriage 17. The lower rightward- Iy-extending portion 17f of load carriage 17 is intended to represent a lateral load-shifting platform. Brackets 17], 17k on the rear of carriage 17 carry a group of conventional mounting rollers which guide the carriage for rolling movement up and down mast members 14a and 14b, and four such rollers are indicated diagrammatically in FIG. 1 at l7a-I7d. The upper right and lower left mounting rollers 17b, are urged by the moment W against upper mast members 14a and 14b, and considering the upper mast section 14 as a unitary body, it will be seen that those rollers apply a clockwise force couple or bending moment to the upper mast section 14, which the upper mast section must resist with an equal opposing moment equal to Rh, where Rh equals W x. In prior art systems using two lift chains, the chains are usually tied to opposite ends of a tensionequalizing bar (not shown) which is pivotally mounted to carriage 17 where the single chain is shown connected in FIG. I, and such an arrangement similarly results in a force couple Rh equal to W x.

The upper mast section 14 is guided in a lower mast section 18 by a set of conventional rollers, four of which are diagrammatically shown in FIG. 1 at l9a-l9d. Upper mast section 14 in turn applies a clockwise bending moment to the next lower mast section 18, comprising members 18a and 18b, which the lower section must oppose by a reaction moment Py, where Py equals W x. If W is re-defined as constituting the payload per se carried on the load carriage, and the term W,, defined as the weight of those portions of the load carriage which are laterally shiftable with the load, the value of the Rh and Py moments will be seen to vary in accordance with the function (W L +W,,,)x. In a twosection mast system, lower members 18a and 18b ordinarily comprise channel members affixed to the base of the truck. In three-section or four-section mast systems, section 18 transmits a bending moment in similar fashion to a pair of stationary members affixed to the truck or via a further pair of extensible members to the pair of stationary members affixed to the truck base. One may note that the force couple applied to any of the mast sections is completely independent of the spacing between the members of the section, and the same force couple is applied by a given laterally shifted load irrespective of whether the mast members of a section are spaced closely together near the centerline of the truck or widely apart near the lateral extremities of the truck. However, in general it is still desirable that the two vertical members of a given mast section be spaced widely apart, since that increases the bending moment of inertia and section modulus of the mast section and decreases the stresses in the mast section. It is well known that the stresses in a beam or similar member vary as: s =M /I where s is the stress of a beam portion distance c from the neutral plane, M is the applied bending moment and I is the moment of inertia of the beam section. U is termed the section modulus. If a I mast section is symmetrical about the centerline 13, its

neutral plane is located at the centerline, of course. In FIG. 1, as a load initially centered between the upright mast members is gradually moved laterallyv outwardly, it will be seen that the lateral bending moments Rb and Py will each increase linearly from zero up to a respective maximum value, as is shown by functions No. l and No. 1a in FIG. 5a. Function No. l arbitrarily assumes that the payload weight W is eight times W,,,, the weight of the laterally shiftable portions of the load carriage. Function No. la assumes a payload 16 times as great as W,,,. Many lift truck lateral load shifting mechanisms can shift a load either rightwardly or leftwardly from a centered position. It is apparent that similar but opposite moments would occur if the load in FIG. 1 were shifted leftwardly. The discussion that follows will tend to be restricted to translation of a load rightwardly from a truck centerline for sake of brevity. It will become apparent that the present invention is applicable both to single-side load-shifting trucks and double-side load-shifting trucks. In the following discussion the term truck centerline will be used, uless otherwise indicated, to denote the centerline between mast members which are spaced apart in the lateral direction, irrespective of the lateral positions of those members relative to the base frame of the truck, although the centerline between the mast members frequently will also correspond to the lateral centerline of the base frame of the truck.

In the basic form of the invention shown diagrammatically in FIG. 2, ram of lift cylinder 11 vertically supports upper mast section 14 in the same manner as in FIG. 1. In FIG. 2, as in the other Figures, the laterally extending lower portion of the load carriage is intended to represent a conventional lateral load-shifting mechanism, several types of which are well known in the art, and hence the details of such mechanisms need not be shown. Four sprockets 21-24 are mounted near the top of telescopic section 14, each journalled to rotate about an axis which extends substantially longitudinally, so that the chains carried by the sprockets extend substantially laterally across the centerline of the truck. Lift chain 25 shown tied to lower mast member 18b at 26 is reeved over sprocket 24 journalled on the right side of mast section 14, substantially horizontally over sprocket 22 journalled at the left side of section 14, and down to be tied to load carriage 17 near its left side, at point 173. A further lift chain 28 shown tied to lower mast member 18a at point 29 is reeved over sprocket 21 which is journalled on the left side of mast section 14, over sprocket 23 journalled on the right side of mast section 14, and down to be tied to carriage 17 near its right side at point 17h. In FIG. 2 chain attachment points 26 and 29 are shown located substantially at the bottom ends of members and 18b. In a twosection mast system such as that shown in FIG. 2, where members 18a and 18b are themselves not elevatable relative to the truck base frame 12, it will be apparent that the attachment points could instead be located on the base frame. If, on the other hand, mast section 16 were an intermediate, elevatable mast section, the chains 25 and 28 are preferably attached near the bottoms of members 180 and 18b, since .the chains do not effectively decrease the bending of portions of the mast section below the attachment points.

In FIG. 2 the sprockets 21 and 24 which route the chains downwardly to attachment points 26 and 29 are shown each located distance c from the centerline, whilethe sprockets 22 and 23 which route the chains downwardly to the load carriage 17 are each shown located at a lesser distance d from the centerline. As will be seen below, the relationship between distances cand d has an effect on the bending moments which result, and the relationship may vary from that shown in various applications of the invention, with c being greater than, less than, or equal to, d in various applications.

When a load on carriage 17 is laterally centered in FIG. 2, ram 10 will provide an upward force equal to twice the weight of the load, and chains 25 and 28 will each be tensioned to half the weight of the load (if one neglects the weights W and W of the non-shiftable and shiftable portions of the load carriage). More precisely, the tension in each chain will equal l/2 (W +W,+ W,,,). Summing vertical forces on load carriage 17:

wherein T is the tension in chain 28, T is the tension in chain 25, W is the weight of the load, W is the weight of those portions of the load carriage which are not laterally shiftable, and W,,, is the weight of the laterally shiftable portions of the load carriage.

Summing moments on load carriage 17, with counterclockwise moments assumed to be positive:

The manner in which the moment Rh which the load carriage applies to mast section 14 will vary as the load is shifted laterally depends upon the closeness of the fit, or the amount of play or backlash between the carriage rollers 17a- -l7d and mast section 14, and upon the spring constant or stretch of chains 25, 28. At least some slight clearance is provided between the carriage rollers and mast section 14 so that the carriage may be raised and lowered without undue friction and binding. It will be seen that at least some slight rotation of load carriage 17 about a horizontal longitudinal axis is necessary for the rollers to seat against section 14, and that the roller forces R, R will be substantially zero until such rotation has occurred. It also can be seen that no such rotation can occur unless the chains have an appreciable amount of stretch.

As the load W L is shifted laterally rightwardly in FIG. 2 through a first distance from the centerline, the roller forces R, R (and hence moment Rh) will be zero, and combining Equations (1) and (2) it will be found that the chain tensions then will vary as follows:

R (w +w,,./2 (1 x/d) w c/2 with the tension in chain 28 increasing and that in chain 25 decreasing. If the chains have insufficient stretch to allow the load carriage to rotate enough for roller forces R, R to occur, such operation with a zero Rh moment may continue out to a load translation distance x ofx, =d (l W /W W,,,), at which point T the tension in chain 25, will be zero and the tension in chain 28 will equal (W W W,). Further rightward translation then causes no change in chain tension, since neither chain can take negative tension, i.e., compression.

Summing vertical forces on a free body comprising load carriage 17 together with telescope section 14: F L m r t L R wherein F is the upward force of ram and W is the weight of telescopic section 14. Substituting (3) and (4) into (5) gives:

F2(W +W,,,+W W,=O

Summing moments on the same free body:

Py +TLC"'TRC (W -i-W X=O By combining (3), (4) and (6) one can find that:

From (7) it may be seen that the Py bending moment applied by the telescopic section 14 to the fixed upright section 18 will be zero throughout such lateral load translation (out to distance x d (l W /W +W,,,) if distance c equals distance d, i.e., if sprockets 21-24 are all mounted equal distances from the centerline, and in some embodiments of the invention the sprockets will be mounted substantially equal distances from the centerline.

Continuing the above assumption of negligible chain stretch, as lateral load translation x exceeds the point at which T becomes zero and T becomes (W W,, W one will find, by substituting those chain ten sion values into Equations (2) and (6), that Rh moment which the carriage applies to section 14, and the Py moment which section 14 applies to the fixed upright section then become:

Rh w, w... (I d) W 1 (WL m) C One may note that the Rh moment depends upon chain spacing distance d, and not upon chain spacing distance 0, and that the Rh moment is decreased by increasing distance d. Also, the Py moment will depend upon both distances 6 and d (if they are unequal) at load translations less than to the point where one chain becomes slack, but that the Py moment becomes independent of distance d after one chain becomes slack, and that the Py moment may be decreased in either case by increasing distance 0. After one chain becomes slack, the Rh moment will be seen to equal the Py moment if distance 0 equals distance d, i.e., if the two sprockets carrying each chain are mounted so as to space the two downward courses of each chain equal distances from the centerline. More importantly, it will be seen that if distances c and d are appreciable fractions of lateral load translation x, the Rh and Py moments are much leas than those which occur in the prior art. By mounting the sprockets as far as is practically possible from the centerline (ram axis), the Rh and Py moments are very significantly reduced. Thus the system of FIG. 2 provides a marked decrease in lateral bending moments over those obtained in prior art structures of the nature of FIG. 1, and further, it provides such a decrease without requiring any complex control system. In FIG. 5a the Rh and Py moments which occur with the system of FIG. 2 are shown as Function No. 2 for the same load used in plotting Function No. I characteristic of the prior art, and shown as Function No. 2a for the same load used in plotting Function No. In characteristic of the prior art, in each case with the assumption that sprocket distances 0 and d of FIG. 2 are equal. The marked decrease in bending moments attributable to the invention is readily apparent from comparison of the functions plotted in FIG. 5a.

While the weight W of the portions of the load carriage which are not laterally shiftable have no effect on the bending moments occurring in the prior art system of FIG. 1, it will be seen from Equations (8) and (9) that the weight serves to further decrease the bending moments which occur in the form of the invention diagrammatically depicted in FIG. 2. In plotting functions No. 2, No. 2a, and No. 2b in FIG. 5a, the weight W of the unshiftable portions of the load carriage was arbitrarily assumed to equal the weight W of the shiftable portions for sake of an example.

If the sprocket distances 0 and d are not equal in the system of FIG. 2, the Rh and Py moments will not be equal. Function No. 2b illustrates the Rh moment which would be obtained if distance d were decreased to percent of distance 0 (to the value shown as d) and the load were the same as that assumed in plotting function No. 2a. The Py moment which would be obtained under such circumstances is shown plotted as function No. 2c, which will be seen to correspond with function No. 2a for values of load translation above that where one chain becomes slack. Conversely, if distance d is made greater than distance 0, with the value shown as d", the Rh moment would vary as function No. 2d, and the Py moment would vary as function No. 2e, which also corresponds to function No. 2a for values of load translation beyond the point where one chain becomes slack.

Viewing lower mast section 18 as a free body in FIG. 2, it will be seen to be subjected to a Py moment applied to it via the rollers interconnecting it with upper mast section 14, which is opposed by an equal reaction between lowermost section 18 and truck base 12. Because chains 25 and 28 are connected at the very bottom of mast section 18, or to the truck base, the tension in the chains will be seen not to apply a moment to section 18, but rather to decrease the roller moment which upper section 14 applies to lower section 18. If the attachment points 26 and 29 of the chains were instead located somewhat upward from where shown, it will become apparent that bending moments equal to those occurring in prior art arrangements would be applied to mast portions below the attachment points. As shown in FIG. 2, the chains will be seen to transmit a portion of the bending moment essentially directly to the truck base, thereby decreasing the bending moments in both upper section 14 and lower section 18. Now, if one imagines the lower end of mast section 18 to be roller-guided in a further lower mast section (not shown) rather than fixedly mounted on the truck vase as shown, it will be readily apparent that the chains still will similarly decrease the bending moments in mast sections 14 and 18, but that the total bending moment of (W +W,,,)x will be applied to the next lower mast section which is not shown. If one imagines instead that an upper mast section (not shown) instead of load carriage 17 is suspended from the chains and guided by section 14, it will be readily apparent that the chains will still decrease the bending of sections 14 and 18, but not decrease the bending of the upper mast section which is not shown. As will be seen below, one or more further chain arrangements similar in principle to that shown in FIG. 2 may be used to similarly decrease the deflection in one or more further lower or upper mast sections. Ultimately, of course, a given amount of lateral load shifting applies a given overturning moment to the truck base irrespective of whether the invention is employed, but the use of the invention provides either much less mast deflection for a given amount of lateral load shifting, or allows use of lighter structural members in the mast, or a combination of both advantages.

The above analysis of the system of FIG. 2 proceeded with the assumption that the chain stretch was insufficient to allow roller forces R, R to occur at any value of rightward load translation less than that value at which T became zero, i.e., chain 25 became slack, and if one makes an opposite assumption, the operation of the system of FIG. 2 is significantly different. If the unequal chain' tensions which occur as the load is translated rightwardly allow sufficient carriage rotation that the carriage rollers seat against the telescopic section 14, so that the roller force R in Equation (2) is no longer zero, the chain tensions no longer change as the load is driven further rightwardly, but tend to remain at the values which they have at the moment when the carriage rollers seat, and further rightward load translation causes increasing roller forces R, R. If the value of load translation at which the carriage rollers seat is designated x;, it can be shown that the Rh and Py moments which begin to occur when the rollers 'seat thereafter vary as:

Rh (WL W,,,)(x x;)

i (W, Wmxx x, c/d) Since these moments exceed those given by Equations (8) and (9), it is apparent that chain stretch should be kept small and/or carriage roller fit kept loose in an embodiment built according to FIG. 2, to insure that the carriage rollers not seat against the telescopic section at load translation values (x) inwardly of that value of x where one chain becomes slack. It can be shown that such operation will be insured if the ratio O /k, is made greater than W W W /2c, where 6, is the amount of carriage rotation required for the carriage rollers to seat against the telescopic section, and k, is the spring constant or stretch of the chains in units of length elongation per unit of force tension. The ratio may be less, it can be seen, for small values of payload W In the mast assembly of FIGS. 3a and 3b the lower mastmembers 18a, 18b are shown as comprising inwardly facing channels. Channels 18a, 18b are assumed to be fixedly mounted to the truck base frame (not shown) in the sense that they are not raised and lowered as the load is raised or lowered, but it is to be understood that they may be arranged to be tilted slightly about a lateral axis, using any one of many conventional mast-tilting arrangements. Rollers 61, 61 journalled on fixed channels 18a, 18b guide extensible uprights 14a, 14b as the latter are raised and lowered. In many embodiments of the invention the roller connection between any two mast sections will be a first set of rollers mounted on the upper mast section near its bot-' tom and a second set of rollers mounted on the lower mast section near its top, so that the first set approach the second set as the upper mast section is raised relative to the lower mast section. Extensible uprights 14a,

14b are shown as comprising conventional I-shapes having their flanges extending laterally. A pair of brackets 17j, 17k extend rearwardly from load carriage 17 to support the carriage in cantilever fashion, and rollers 62, 62 carried on stub shafts mounted on brackets 17j, 17k roll against inner flangesof uprights 14a, 14b to guide carriage 17 as it moves vertically. The cantilever suspension of the load carriage applies a longitudinal vending moment to uprights 14a, 14b via the circumferential surfaces of rollers 62, 62 even when the load is laterally centered. Further rollers 63, 63 carried on telescopic members 14a, 14b correspond in principle to rollers 19a-19d of FIG. 2 and transmit forces between telescopic members l4a, 14b and fixed uprights 18a, 18b when the load is sufficiently laterally displaced.

The upper ends of extensible uprights 14a, 14b are shown interconnected by a short member 64 and by plate 65, which correspond in principle to crosspiece in FIG. 2. The upper end of hydraulic ram 10, which in some embodiments might instead comprise a pair of rams, is shown for sake of simplicity as being merely bolted to crosspiece 65 by means of bolts The ram or rams may connect to the crosspiece in any wellknown fashion, using a pivotal connection, for example.

A pair of brackets 66a, 66b welded to the outer flanges of extensible upright 14a support shaft 67 upon which sprockets 21 and 22 are rotatably mounted, and similar brackets 68a, 68b welded to the outer flanges of extensible upright 14b similarly support shaft 69 upon which sprockets 23 and 24 are rotatably mounted. One end of roller chain 25 is tied to load carriage 17 by means of arm 17g which extends rearwardly from carriage 17. Chain 25 then passes upwardly over sprocket 22, then substantially horizontally to sprocket 24 through openings 14c, 14d provided in the webs of uprights 14a, 14b, and then downwardly to terminate at attachment means 26 affixed either to fixed upright member 18b near its lower end, or to the truck base. One end of roller chain 28 is tied to load carriage 17 by means of arm 17h which extends rearwardly from carriage 17. Chain 28 then passes upwardly over sprocket 23, then substantially horizontally through openings 14d, 140 to sprocket 21, and then downwardly to terminate at attachment means 29 affixed either to the truck base or to fixed upright 18a near its lower end.

It may be noted that the pair of sprockets shown mounted adjacent the top of each extensible upright are shown with differing diameters. Since each chain must have one of its ends connected to the load carriage and its other end connected to the fixed upright on the opposite side of the mast, it will be seen that unless the two chains are arranged to cross each other in a longitudinal sense, an arm must extend from load carriage 17 past the forward chain (25 in FIG. 3a) to the rear chain, or the chains may slope at angles other than the vertical. Thus sprocket 23 is provided with a larger diameter than sprocket 24 so that arm 1711 is situated slightly laterally outisde the downward course of chain 25 and will clear chain 25. Because arm 17g need not extend to the rearward chain, there is no actual need in FIG. 3a that sprocket 22 have a larger diameter than sprocket 21 to locate arm 17g laterally outwardly from attachment means 29, but in FIG. 3a such an arrangement has been shown so that the downward courses of chains 25 and 28 to attachment means 26 and 29 are laterally equidistant from ram 10. In FIGS. 3a and 3b, it will be seen that the use of two different diameter sprockets results in a slight difference between the distances c and d, with distance d exceeding distance in FIGS. 3a and 3b.

It will become apparent that one need not use two different diameter sprockets on shaft 69 to allow arm 17h to extend past the forward chain 25. Instead, one may provide two separate shafts laterally displaced from each other by the desired lateral distance between the two chains down that side of the mast, and use sprockets of the same diameter on two such shafts. Such an arrangement is shown diagrammatically in FIG. 30, where shaft 69a carrying front sprocket 24 is shown located upwardly as well as leftwardly from rear sprocket 23 carried on shaft 69b, so that shaft 69a will clear sprocket 23' and shaft 69b will clear sprocket 24. If the two sprockets on a given side of a mast are mounted on separate stub shafts which do not extend completely between the two flanges of member 14b, it will be apparent that the two shafts may lie at the same vertical level. Furthermore, neither different size sprockets nor displaced sprocket shafts need be used to route the chains if the chains slope at non-vertical angles, and as will be seen below, some embodiments of the invention use chains which slope at distinctly nonvertical angles.

FIGS. 3d and 3e diagrammatically illustrate an alternate arrangement wherein the two chains 25 and 28 are arranged to extend at angles to the lateral direction so that the horizontal courses of the chains cross, one chain being slightly above the other (enough above so that it does not droop and rub on the other chain when it goes slack). The downward courses of chain from rear sprockets 21 and 24 may connect to lower uprights 18a and 18b, respectively, or to the truck base, and the downward courses from sprockets 22 and 23 may connect to arms 17g and 17h which extend rearwardly from the load carriage. It may be noted that distances c and d are shown equal to each other in FIGS. 3d and 32. It will be apparent that the arrangement of FIGS. 3d and 32 will readily allow the designer freedom to make either one of these distances greater than the other if desired.

The concept utilized to reduce lateral bending moments in the system previously described in connection with FIG. 2 may be advantageously combined with a prior proposed system using dual lift cylinders to further reduce lateral bending moments, as is shown in connection with FIG. 4, wherein parts basically similar in principle are given similar reference numerals. Rather than being supported by a single lift cylinder, the telescopic mast section 14 in FIG. 4 is supported by two separate hydraulic cylinders 11a, 11!) which are laterally spaced widely apart, each preferably near one lateral extremity of the vehicle. In FIG. 4 the chain spacing distance 0 is shown exceeding the distance d, with the ram spacing distance r in between the distances c and d, but it will become apparent that those specific distance relationships are not essential. As in FIG. 2, chain 25 extends from attachment point 26 upward, over pulleys 24 and 22 and down to carriage 17 at 17g, while chain 28 extends from attachment point 29 upward, over pulleys 21 and 23 and down to carriage 17 at point 17h. In one embodiment constructed according to FIG. 4, the two lift cylinders 11a, 11b are both supplied with hydraulic fluid from a main lift pump when most lifting is done, with each cylinder being connected to the lift pump through a separate respective check valve. When fluid is supplied at a given pressure to both lift cylinders from the same lift pump, and the load has not been translated out beyond where one chain has become slack, the two cylinders act in unison in the same manner that an equivalent single cylinder located at the centerline would act, as in FIG. 2, and hence zero Rh and Py moments occur if distance 0 equals distance d. If lifting is done with both cylinders when the load has been translated outwardly beyond the point where chain 25 has become slack, the Py moment will vary as:

y (WL mX 4 Variation of the Rh moment with lateral load translation x is shown as Function No. 3 in FIG. 5b, and variation of the Py moment when lifting with both cylinders is shown as function No. 2, with distances r, d, c and weights assumed as indicated in FIG. 5b. When no lifting is occurring, both check valves are held closed by the weight applied to the lift cylinders, and if a load is then translated laterally rightwardly, the pressures in the two cylinders automatically vary, the right cylinder pressure increasing and the left cylinder pressure decreasing while the two chain tensions simultaneously change, one increasing and the other decreasing. During this mode of operation, the change in the chain tensions keeps the Rh carriage-to-telescopic section bending moment essentially zero and the change in cylinder pressures or forces keeps the Py telescopic-to-fixed upright section bending moment zero. Assuming little chain stretch and/or ample carriage roller clearance, this mode of operation could continue for outward load translations (increasing x) until either chain 25 becomes slack or the force on and pressure in left cylinder 11a became zero. Although a load certainly may be shifted outwardly beyond a point where one chain becomes slack, it is important that neither cylinder force becomes zero, in order to avoid the introduction of air into the hydraulic system. The system of FIG. 4 preferably includes automatic control means (not shown) which operates to relieve the leftside cylinder 11a down to a low pressure just sufficient to keep it adequately charged with oil, as a load is shifted sufficiently far rightwardly. When the leftside cylinder is relieved down to a low pressure, substantially the entire weight of the load is borne by the rightside cylinder. Since the effective support point is then shifted outwardly to the right cylinder, the Py bending moment is further reduced. If the leftside cylinder 11a is relieved to a low pressure designated F,,, the Py moment obtained will vary as:

if neither chain is slack (which will be seen to render it invariant with load translation if the two sprockets carrying each chain are equidistant from the centerline, i.e., c d), and varies as:

upon further lateral load translation past the point x at which chain becomes slack. The variation of the Py moment with load translation is shown as function No. 4 in FIG. 5b for the same conditions assumed for the other functions of FIG. 5b, and with the additional assumption made that the leftside cylinder was relieved to a pressure such that its force F was equal to carriage weight-W The effect of distances 0, d and r on bending moments is readily deducible from the equations given.

It will be apparent that a lift cylinder may be relieved down to any desired pressure by connecting it to the hydraulic oil sump (assumed to be at atmospheric pressure) through a relief valve set to the desired relief pressure. It will also be apparent that relieving one cylinder may be done either automatically, such as by electrical switches responsive to carriage lateral shifting, or manually by the operator, and since details of the hydraulic control system form no part of the present invention, they need not be shown herein. It should be noted that the crossed-chain arrangement of FIG. 4 may be used with dual lift cylinder systems irrespective of whether they incorporate means for relieving one cylinder, and irrespective ofwhether the cylinders use separate check valves to allow the cylinder forces to differ.

It has been demonstrated above how the chain system of FIG. 2 will reduce the lateral bending moment which alaterally-displaced load otherwise would cause in the upper mast section 14 and in all portions of the adjacent lower mast section 18 above the chain attachment points 26, 29 to the lower mast section. In FIG. 6 the lower portion of an upper mast section 14 is shown, and mast section 14 is assumed to be connected to a load carriage (not shown) and to lower mast section 18 in the manner shown in FIG. 2, and is assumed to be raised and lowered by ram 10 in the same manner as in FIG. 2. In FIG. 6 mast section 18 is illustrated as comprising an intermediate mast section including members 18a, 18b which are guided by members 32a, 32b of afixed mast section 32, the latter member being affixed to base frame 12 of a truck. To reduce the lateral bending moments in mast sections 18 and 32, a chain arrangement similar in principle to that employed in FIG. 2 may be used. Chain 33 is affixed at one end near the lower end of lowermost mast members 32b, (or directly to base 12 as shown at 34 if mast section 32 is not elevatable), at distance e from the centerline in either case. The chainthen passes upwardly and over sprockets 35 and 36 carried by intermediate mast section 18, and attaches to upper mast member 14a at attachment point 37 distance f from the centerline. Similarly, chain 41 is affixed at one end near the bottom of lowermost mast member 32a at 42, then passes upwardly and over sprockets 43 and 44 carried on section 18, and then attaches to upper mast member 14b at 45. When the load platform (not shown) has been shifted laterally rightwardly in FIG. 6 beyond a given distance the load carriage (not shown) and upper mast section 14 will apply a clockwise Py moment to mast section 18 in the manner described above in connection with FIG. 2. Without the illustrated arrangement of chains 33 and 41, the fixed uprights of mast section 32 would receive and have to resist an Mz lateral bending moment equal to the Py moment. With chain arrangement shown, the Py moment will tighten chain 41 and slacken chain 33. The tension in chain 41 will be seen to transmit a portion of the Py moment from section 14 directly to the truck base, thereby decreasing the bending moments in both sections 18 and 32 of the mast.

The manner in which the M2 bending moment will vary with load translation also may depend upon roller clearance and chain stretch. First assume chains 33 and 41 have little stretch compared to the clearance of the rollers which interconnect mass sections 18 and 32. As a load is translated rightwardly the chain tensions initially vary as:

where T and T are the tensions in chains 41 and 33, respectively. The M2 moment then will vary as follows:

w, +w x 1 2e/j) It will be seen that the M2 moment will be zero if sprocket distance e equals one-half of sprocket distance f. Otherwise the M2 moment will vary in one direction or the other, similar to the manner in which the Py moment in FIG. 2 depends upon the relationship between sprocket distances c and d. Thus in some em bodiments of the invention, the sprockets which route the chains to the telescopic section uprights will be spaced farther from the centerline than those which route the chains to the base. Such an arrangement is shown in FIG. 6a. The above variation of the Mz moment occurs from a centered load position out to a position of:

x W 0.5 WJW "l W the lateral load position at which chain 33 has zero tension and chain 41 supports the entire load of (W W,, W W,). Upon further laterally outward load translation the M2 moment will vary as follows, analogously to Expression (9):

Mz (W W x Inasmuch as operation in accordance with Expression (17) provides a significantly smaller Mz moment if distance e is appreciable, it is deemed advisable to provide ample roller clearance and little chain stretch in chains 33 and 41 in embodiments constructed in accordance with FIG. 6.

The arrangement shown in FIG. 6 is unusual in that ram connects to the top of section 14. In ordinary applications that would require that it comprise a double-stage ram, in order that it be capable of extending that far upwardly. Instead, in more usual applications, ram 10 will comprise a single-stage ram which connects to a crosspiece interconnecting members 18a and 18b near their tops, as shown in FIG. 6a, so that the ram will raise and lower mast section 18. The load carriage then will elevate at three times the ram velocity and the ram force F required to support the load then will equal:

where W,, is the weight of mast section 18.

Various ways in which the invention may be applied to multiple section masts will become apparent from the diagrams of FIGS. 6b, 6c and 6d. Only the chains which bear the load when the load has been shifted far leftwardly are shown in these Figures for clarity and ease of illustration. FIG. 6b shows a two-section mast and corresponds in principle to FIG. 2. Ram 10 raises upper mast section 14 at the ram velocity, thereby raising carriage 17 at twice the ram velocity. The maximum distance through which carriage 17 may be raised and lowered equals twice the distance through which ram 10 may be extended and retracted. Lift chain 25 extends from attachment point 17g on carriage 17 over sprockets 22 and 24, and is tied to base frame 12 (or, if desired, to lower member 18b preferably near its lower end) at attachment point 26. The force transmitted by chain 25 directly to base 12 decreases the bending moments in both upper mast section 14 and lower mast section 18.

FIG. shows a three-section mast utilizing two serially acting hydraulically interconnected lift cylinders, 11a and 1 1b. Lower cylinder 11b acts between base 12 of the truck and the crosspiece top 18c of intermediate mast section 18. Upper cylinder 11a acts between crosspiece or base 14d of upper section .14 and an auxiliary mast section 12' which is guided for vertical movement in between members 14a, 14b of upper mast section 14. Sprockets 22 and 24 are mounted on auxiliary section 12'. Starting from a fully collapsed or retracted condition, upper cylinder 11a raises carriage 17 to the top of upper mast section 14, with the carriage raising auxiliary section 12 at the extension velocity of cylinder 11a and thereby raising carriage 17 at twice that velocity. When auxiliary section 12' or carriage 17 reaches stops at the top of section 14, increased hydraulic pressure then causes lower cylinder 11b to extend. Cylinder 11b raises intermediate mast section 18 at the velocity of lower cylinder 11b, thereby raising upper mast section 14 and carriage 17 at twice the velocity of lower cylinder 11b. The maximum distance through which carriage 17 may be moved equals two times the maximum extension of upper cylinder 11a plus two times the maximum extension of lower lift cylinder 11b. Lift chain 25 extends from attachment point 17g on carriage 17 over sprockets 22 and 24, and is tied to base member 14d of upper mast section 14 at point 26. The force transmitted via chain 25 will be seen to reduce the bending moments in both mast section 14 and auxiliary section 12. To similarly reduce bending moments in sections 18 and 32, lift chain 33 extends from attachment point 37 on base 14d of mast section 14, over sprockets 36 and 35, and is tied to the base frame 12 at the lower end of stationary mast section 32 at point 34.

FIG. 6d diagrammatically illustrates one application of the invention in connection with a different threesection mast system which uses a single cylinder to raise and lower three times the cylinder extension distance. Lift cylinder 11c and its ram 10 act between the truck base frame 12 and the top of intermediate mast section 18%. As hydraulic pressure raises cylinder 11c and section 18 at the cylinder extension velocity, upper telescopic mast section 14 is raised at twice that velocity by chain 33, and carriage 17 is raised at three times that velocity by chain 25. Chain 25 extends from attachment point 17g on carriage 17 over sprockets 22 and 24 and is tied to the base of intermediate mast section 18 at point 26, and it will be appreciated that the chain 25 arrangement reduces the bending moments in mast sections 14 and 18. Chain 33 extends from attachment point 37 on section 14 over sprockets 36 and 35 and is tied to the truck base 12 at point 34, and it will be appreciated that the chain 33 arrangement reduces the bending moments in mast sections 18 and 32. It will become apparent that the concept shown in FIG. 6d can readily be extended by the addition of one or more further mast sections. For example, rather than connecting to the load carriage 17, the upper end of chain 25 could be connected to the base of a further telescopic section (not shown) guided between the members 14a, 14b of upper section 14, the load carriage 17 mounted on that further telescopic section, and a further chain routed from the load carriage, over a sprocket adjacent the top of the further section, to an attachment point preferably near the bottom of section 14, and, of course, a further similar chain would be provided on the other side of the truck.

It should be noted that the invention is applicable to what is generally termed a single-section mast. For example, member 14d in FIG. 60 could comprise the base frame of a truck rather than being elevatable as shown, and mast section 14 then comprise a fixed upright mast section. What is termed auxiliary section" 12' in FIG. 60 will be seen not to' comprise a mast section in the usual terminology. The chain 25 arrangement shown in FIG. 66 (and a similar chain routed oppositely, of course) may interconnect the truck base and the load carriage as shown in FIG. 6c, where the ram associated with a single-section raises and lowers pulleys 22, 24 (and similar pulleys for the other chain).

While the basic concepts of the invention have been illustrated in connection with various lift cylinder configurations and chain arrangements, it is important to recognize that the invention is applicable to a variety of other known forms of truck masts, irrespective of how many mast sections are used and how many lift cylinders are stacked vertically, and irrespective of how the chains are routed to provide the desired lifting speed. Basic principles of the invention are applicable to any truck mast having (I) an elevatable mast section, (2) a device guided for vertical movement by either that mast section or a further mast section, whether the device be a load carriage or a further mast section, and (3) the further mast section arranged to guide the elevatable mast section, whether or not the further mast section itself be elevatable.

In each of FIGS. 6b-6d chain 25 is shown connected near the top of load carriage 17, but it will be apparent that it need not be, and chain connections may be made at other vertical locations on the load carriage. The lat-v eral bending moments occurring within the load carriage are usually insignificant due to its limited height.

While the invention, as thus far discussed, has been illustrated in connection with the most widely used form of mast, wherein each successively higher telescopic mast section is situated laterally in between the adjacent lower mast section which guides it, it is important to note that the invention is in principle applicable as well to what is sometimes termined an inside-out mast assembly, wherein each successively higher mast section is situated laterally outside the adjacent lower mast section which guides it. A mast system of such a type incorporating one form of the invention is shown diagrammatically in FIG. 7. Chain 25 extends upwardly from attachment point 17g on carriage 17 over sprocket 22 then laterally rightwardly over sprocket 24 and downwardly to attach to member 18b of the lower mast section, preferabl-y'near the lower end thereof. Similarly, chain 28 extends upwardly from attachment point 17h on to carriage 17 over sprocket 24, then laterally leftwardly over sprocket 22 and downwardly to attach to member 18a of the lower mast section, preferably near the lower end thereof. Because the downward courses of chain which attach to the lower mast section are shown connected to the inner sides of the lower section members, and because the lower mast section members in an inside-out system generally will be spaced closer together than those of a regular outsidein system for a truck of given width, it will become apparent that the arrangement of FIG. 7 lends itself to embodiments of the invention wherein it is desired that dimension d be substantially larger than dimension 0. The Rh and Py moments which occur with such a system are the same as those shown above in connection with FIG. 2 where dimension d exceeds dimension c.

In the embodiments of the invention described thus far, the chains which extend downwardly from sprockets on a given mast section to the load carriage and to members of the adjacent lower mast section will have all been shown as being trained vertically straight down. Various modified embodiments may utilize slanting non-vertical chain courses in various applications to achieve various beneficial effects. Referring now to FIG. 8, which is a modified version of FIG. 2, the load W is shown shifted leftwardly so that chain 25 bears the entire load. A second chain which is routed over two more sprockets (not shown) in mirror-image fashion is slack, and is not shown in FIG. 8. It may be noted that sprockets 22 and 24 carrying chain 25 are mounted at distinctly different heights on upper mast section 14. That difference in height has no appreciable effect on the lateral moments produced. However, the connection of chain 25 to carriage 17 other than directly below the edge of sprocket 22 will provide a significant effect. In FIG. 8 where chain 25 supports the total vertical weight, it may be seen that the vertical component of the tension T in chain 25 must equal L, where L equals (W W, W,,,). The vertical component of tension T will be seen to equal T sin A, where angle A is the slope of chain 25 between the upper mast section and load carriage 17. Thus T L/sin A. It will be apparent that angle A will change as the load carriage is raised and lowered. If angle A becomes very small as carriage l7 rises, it will be seen that sin A will become very small and the tension T will increase very greatly. In FIG. 8 the moment Py applied to mast section l8 will be seen to comprise the difference between a CCW moment (W W,,,)x and a CW moment of (W W W c/sin A,

Cl-lCm)(x c/sin A) W c/Sin A Thus slanting the chain may significantly increase tension T and thereby increase the CW moment, thereby providing a decreased total moment, or even changing the sign of the total moment. As will be seen below, sprocket 22 may be (and preferably is) eliminated, and chain 25 is routed directly from sprocket 24 to an attachment point such as 94, via the dashed line shown at 25'. It should be noted that carriage 17 in FIG. 8 

1. A material-handling vehicle mast assembly having reduced lateral bending moments in the vertical structural members of the mast sections of said assembly for loads extended laterally beyond the lateral extremities of said structural members, comprising, in combination: a first mast section; a second mast section comprising a pair of vertically extending structural members rigidly connected to each other and spaced apart in a lateral first direction for guiding vertical movement of said first mast section; a first plurality of rollers situated between said first and second mast sections to rollingly transmit forces from said first section laterally to said second section, each of said rollers of said first plurality being rotatable about a respective axis extending in a longitudinal direction substantially perpendicular to said lateral first direction; vertically extensible ram means for supporting said mast assembly at an effective support point and for providing relative vertical movement between said first and second mast sections; a third mast section including means for supporting a load; a second plurality of rollers situated between said third mast section and one of said first and second sections to rollingly transmit forces from said third section laterally to said one of said sections, each of said rollers of said second plurality being rotatable about a respective axis Extending substantially in said longitudinal direction; first pulley means carried on said first section; first cable means connected from a first point on said third section which lies on a first side of said support point and extending generally in said first direction over said first pulley means to said second section at a second point attached to said second section, said second point lying on the side of said support point opposite from said first side; second pulley means carried on said first section; second cable means connected from a third point on said third section which is on said opposite side of said support point, over said second pulley means, to a fourth point attached to said second section and lying on said first side of said support point; and means supported by said third mast section for shifting said load in said first direction between a first range of positions at which the center-of-gravity of said load lies laterally between said first and third points and both of said cable means are taut, to a second range of positions at which said center-of-gravity does not lie laterally between said first and third points, one of said cable means becomes slack, and the weight of said third mast section and said load is borne by the other of said cable means.
 2. The combination according to claim 1 wherein said third section of said mast assembly comprises a load carriage.
 3. The combination according to claim 2 wherein said load carriage is guided for vertical movement by said first section of said mast assembly.
 4. The combination according to claim 2 wherein said load carriage is guided for vertical movement by said second section of said mast assembly.
 5. The combination according to claim 4 wherein said portions of said cable means are vertically displaced from each other.
 6. The combination according to claim 1 wherein said third section of said mast assembly comprises a vertically extensible mast section having a load carriage suspended therefrom by means of further cable means.
 7. The combination according to claim 1 wherein said first pulley means comprises first and second pulleys located on said first side and said opposite side, respectively, of said support point.
 8. The combination according to claim 1 wherein said first pulley means comprises a pulley mounted on said first section on said opposite side of said support point and said first cable means extends directly between said pulley and said first point on said third section.
 9. The combination according to claim 1 having a truck base, said second section of said mast assembly being fixed against vertical movement relative to said truck base.
 10. The combination according to claim 1 wherein said second point is located on said second section.
 11. The combination according to claim 1 having a truck base, said second section being fixed to said truck base and said second point being located on said truck base.
 12. The combination according to claim 1 wherein one end of said extensible ram means is connected to pivotally engage said first section and the other end of said extensible ram means is connected to said second section.
 13. The combination according to claim 1 wherein said extensible ram means comprises a plurality of extensible rams spaced apart in said first direction.
 14. The combination according to claim 1 wherein siad first cable means extends through a vertical reference plane which extends normal to said first direction and passes through said support point.
 15. The combination according to claim 1 wherein said second point is located at a greater distance in said first direction from said support point than the distance in said first direction of said first point from said support point.
 16. The combination according to claim 1 wherein said second point is located at a lesser distance in said first direction from said support point than the distance in said first direction of said first point from said support point.
 17. The comBination according to claim 1 wherein said ram means comprises a pair of hydraulic rams spaced apart in said first direction and said combination includes means for reducing pressure in one of said hydraulic rams to shift said effective support point in said first direction.
 18. The combination according to claim 1 wherein said first cable means extends substantially vertically between said first point and said first pulley means.
 19. The combination according to claim 1 wherein said first cable means extends substantially vertically between said first pulley means and said second point.
 20. The combination according to claim 1 wherein said first point is offset in said first direction from below said first pulley means, whereby the slope of the portion of said first cable means between said first point and said pulley means varies as the vertical position of said first section varies relative to the vertical position of said third section of said mast assembly.
 21. The combination according to claim 1 having means for automatically varying the location of said first point on said third section of said mast assembly.
 22. The combination according to claim 21 in which said means for varying the location of said first point is operative to vary said first point as a function of the vertical position of said third section of said mast assembly relative to said first section of said mast assembly.
 23. The combination according to claim 21 in which said means for varying the location of said first point is operative to vary said first point as a function of the translation in said first direction of said load from said support point.
 24. The combination according to claim 21 in which said means for varying the location of said first point is operative to move said first point vertically on said third section.
 25. The combination according to claim 21 in which said means for varying the location of said first point is operative to move said first point horizontally on said third section.
 26. The combination according to claim 21 in which said means for varying the location of said first point is operative to move said first point both vertically and horizontally on said third section.
 27. The combination according to claim 1 having third pulley means mounted at said second point, and wherein said first cable means extends from said first pulley means to said third pulley means and extends from said third pulley means to attach to a third point located on said first section on said opposite side of said support point.
 28. The combination according to claim 1 having a truck base and wherein said extensible ram means is connected to raise said second mast section relative to said truck base, second pulley means mounted on said second mast section, and second cable means connected from said first section over said second pulley means to said truck base, whereby extension of said ram means raises both said first and second mast sections relative to said truck base.
 29. The combination according to claim 1 wherein said first mast section comprises a pair of vertically-extending structural members rigidly connected to each other and spaced apart from each other in said first direction.
 30. The combination according to claim 1 wherein said vertically extensible ram means is located midway in said first direction between said vertically extending structural members.
 31. The combination according to claim 1 wherein said vertically extensible ram means is displaced in said first direction nearer to one said vertically extending structural members than to the other of said vertically extending structural members.
 32. The combination according to claim 1 wherein said means supported by said third mast section for shifting said load comprises means operable to translate said load in said first direction to move said center-of-gravity of said load away from said support point to a position beyond said first point on said first side or to a position beyond saId third point on said opposite side.
 33. The combination according to claim 1 wherein portions of said first and second cable means extend substantially parallel to each other in said first direction on both sides of said support point.
 34. The combination according to claim 1 wherein portions of said first and second cable means in between said first and third points cross each other in a second direction perpendicular to said first direction.
 35. The combination according to claim 1 wherein said second plurality of rollers are situated between said third mast section and said one of said mast sections to provide a predetermined clearance, said clearance being made sufficiently great in relation to the stretch in said cable means that upon progressive lateral shifting of said load away from said effective support point one of said cable means becomes slack before said clearance is taken up and said rollers of said second plurality thereafter transmit said forces laterally to said one of said sections.
 36. A material-handling vehicle mast assembly having reduced lateral bending moments in the vertical structural members of the mast sections of said assembly for loads extended laterally beyond the lateral extremities of said structural members, comprising, in combination: first and second mast sections each comprising a pair of vertically extending structural members rigidly connected to each other and spaced apart in a lateral first direction, said first and second mast sections being disposed in a telescoping relationship with said second section guiding vertical movement of said first section; vertically extensible ram means for supporting said first mast section at an effective support point and for providing relative vertical movement between said first and second mast sections; load carriage means; a plurality of rollers situated between said load carriage means and said first mast section with a clearance to rollingly transmit lateral forces from said load carriage to said first section upon rotation of said load carriage means to take up said clearance; first and second pulley means carried on said first section; first cable means connected from a first point on said load carriage on a first side of said support point laterally over said first pulley means to a second point attached to said second section on the side of said support point opposite from said first side; second cable means connected from a third point on said load carriage means on said opposite side of said support point over said second pulley means to a fourth point attached to said second section on said first side of said support point; and means carried on said load carriage means for laterally shifting a load carried on said load carriage means, said clearance being sufficiently great in relation to the stretch of said cable means that upon increasing lateral displacement of said load over a first range of positions from said effective support point the tensions in said cable means change and said rollers do not apply substantial lateral forces to said first mast section, and upon further lateral displacement within a second range of positions one of said cable means becomes slack and said rollers apply lateral forces to said first mast section. 